The present invention relates to a process chamber for processing semiconductor substrates.
In the manufacture of integrated circuits, active devices are fabricated on a semiconductor substrate by alternately depositing and etching layers of dielectric, semiconducting, or conducting materials, such as silicon dioxide, polysilicon, metals and their silicides or nitrides. The etching process is performed using an energized halogen or other gas, such as for example, CHF.sub.3, CF.sub.4, BCl.sub.3, HCI, O.sub.2, NF.sub.3, N.sub.2, or Ar, which is introduced into the chamber by a gas distributor comprising holes for distributing the gas in the chamber. Conventional gas distributors are made of polycrystalline ceramic or metal that often rapidly erode in erosive process gas to form contaminant particles that deposit on the substrate. For example, gas distributors made of polycrystalline alumina erode in fluorine-containing gases, while aluminum gas distributors erode in chlorine-containing gases. It is desirable to have a gas distributor that is resistant to erosion in highly reactive process gases.
Another problem with conventional ceramic gas distributors arises because the brittle ceramic material often cracks or chips during machining of the gas distributor structure and its holes. Furthermore, it is difficult to fabricate a ceramic gas distributor having a sufficiently large diameter to uniformly distribute gas across large semiconductor substrates having diameters of up to 300 mm. In addition, polycrystalline ceramic gas distributors contain fine grains having diameters of 0.1 to 50 microns that often form rough edges at the holes and surfaces of the ceramic gas distributor. The ceramic grains and their grain boundary regions also contain impurities which are eroded by the process gas, causing the ceramic grains to flake off and contaminate the substrate. Another problem arises from the high thermal expansion and an uneven heat load inside the process chamber that causes the center of the gas distributor to be hotter than its perimeter causing thermal stresses, which in turn cause cracks, flaking, and breakage of the gas distributor.
Yet another problem arises from the large thermal mass of conventional gas distributors which can result in a "first wafer effect" in which the first few substrates are effectively processed at different processing rates than subsequently processed substrates, even though the same temperature, pressure, gas flow and other process conditions are set to predetermined levels in the chamber. In etching processes, it is believed that the first substrate processed in the chamber is processed at a lower etch rate than the subsequently processed substrates because of the presence of unstable gaseous species initially formed in the activated gas or plasma. An alternative explanation for the first wafer effect is that the substrates are processed at different temperatures because the gas distributor has a large thermal mass and absorbs or releases heat at unusually high rates during processing of the first substrate as compared to subsequent substrates processed. It is desirable to have a process chamber that provides little or no first wafer effect, and it is further desirable to have a gas distributor with a low thermal mass to reduce thermal fluctuations in the chamber to provide more consistent processing rates of the substrates.
Yet other problems with conventional ceramic gas distributors arise because the polycrystalline ceramic used to form the distributor is not transparent to light and blocks transmission of light beams used by an endpoint detection system. Optical endpoint techniques are used to monitor the endpoint of a process being performed on a substrate. A preferred optical endpoint technique is laser interferometry in which a laser beam is reflected off the substrate surface, and interference between the different portions of the laser beam which are reflected from the top and the bottom of a transparent layer on the substrate are used to monitor etching of the transparent layer, as for example, described in U.S. Pat. No. 4,953,982, issued Sep. 4, 1990, which is incorporated herein by reference. In laser interferometry, it is desirable to direct the laser beam perpendicular to the surface of the substrate for endpoint detection during etching of layers having high aspect ratio trenches because a low angle laser beam is blocked by such trenches. However, polycrystalline ceramic gas distributors are opaque and prevent transmission of the laser beam therethrough. It is known to align a hole in the gas distributor with the laser beam to pass the laser beam through the hole and into the chamber, however, it is difficult to assemble a process chamber, gas distributor hole, and laser beam apparatus with the necessary degree of alignment. Also, the gas distributor holes have to be sized sufficiently wide to allow a laser beam to be reflected to and from the substrate and through the hole. However, the wide diameter hole often does not allow uniform distribution of process gas into the chamber. Also, directing a laser beam through a small hole precludes scanning of the laser beam across the substrate to find a suitably flat or transparent surface on the substrate on which to make an endpoint measurement. Thus it is desirable to have a gas distributor that transmits a light beam therethrough and preferably allows the light beam to be transmitted perpendicularly to the substrate surface for end-point detection systems.
Another problem occurs in plasma process chambers where the ultraviolet, visible, or infrared light emissions generated by the plasma are reflected from the non-transparent or opaque ceramic gas distributor (which typically forms the ceiling of the chamber and faces the substrate) and are non-uniformly incident on the substrate. The light emissions may enhance etching of localized spots on the substrate where a high energy of light is incident, resulting in non-uniform etching rates at different points across the surface of the substrate. It is desirable to have a gas distributor that is transparent to the energetic electromagnetic transmissions generated by the plasma to allow these emissions to pass more uniformly through the gas distributor instead of being reflected toward the substrate.
Accordingly, it is desirable to have a process gas distributor that is capable of providing a uniform gas distribution in a process chamber. It is further desirable for the gas distributor to exhibit low rates of erosion in halogen plasma environments. It is also desirable for the gas distributor to allow thermal expansion stresses without cracking or breakage during processing. It is further desirable for the gas distributor to transmit a light beam of an optical endpoint detection system, and to be transparent to energetic electromagnetic transmissions generated by a plasma in the chamber.